The present invention relates generally to high frequency amplifiers, and in particular to means for decreasing pulse recovery time inherent in an AC coupled unipolar compression video amplifier (CVA).
As is well known to those skilled in the art, if an input were DC coupled to an amplifier, there would generally be no pulse undershoot or recovery problem. However, continuous wave (CW) signals are commonly present along with pulse signals and are operated upon by DC coupled amplifiers. Accordingly, CW and pulse signals are linearly superimposed and amplified together causing difficulty in determining true pulse amplitudes, a phenomenon which is usually referred to as "jammed." Long pulses have a similar effect on amplifiers designed for narrow pulse reception.
The usual technique for eliminating CW from the incoming signal is to pass the signals through an AC coupling (a resistor-capacitor) network before applying it to the amplifier. AC coupling achieves the desired DC blocking, but necessarily introduces a "droop" into the pulse envelope starting at the leading edge, and a trailing edge undershoot equal to the droop. A pulse amplifier must recover from the trailing edge undershoot of a pulse before the next pulse arrives, or that next pulse will be offset by the undershoot. Recovery from undershoot is generally at the same rate as the droop, both being determined by the RC time constant of the AC coupling network. An increase in the RC time constant decreases the rate of droop and the amplitude of the undershoot, but decreases the rate of recovery. Output constraints often limit acceptable droop to about 8%. This means that for narrow (1 .mu.sec) pulses there will be time for sufficient pulse recovery for pulse trains up to about 7000 pulses per second. For wider pulses, sufficient pulse recovery time requires a lower pulse per second rate. If the period of the pulses is much longer than their duration, then recoveries will be practically complete, and undershoot will not be a problem.
Pulse trains up to one million pulses per second are now being used in, for example, sophisticated radar systems. Accordingly, there is a need for amplifiers capable of recovering faster than amplifiers known heretofore to be able to handle these high pulse rates which require a recovery time for each pulse fast enough for not giving erroneous amplitudes for the next pulse. It is required that a small signal, just above the receiver's sensitivity threshold, be received unimpaired in amplitude even when a pulse with maximum amplitude and hence large undershoot and delayed recovery has immediately preceded it. Because of their relatively slow pulse recovery time, conventional AC coupled CVA's are unsatisfactory.